Solid carbonaceous subterranean formations such as coal formations contain significant quantities of hydrocarbon gases, usually including methane, trapped therein. These gases represent a valuable resource if they can be produced economically. Where such a formation is to be mined later, it is also beneficial from a safety standpoint to produce as much of these gases as possible before commencement of mining operations. The majority of such gas, however, is sorbed onto the carbonaceous matrix of the formation and must be desorbed from the matrix and transferred to a wellbore in order to be recovered. The rate of recovery at the wellbore typically depends on the gas flow through the solid carbonaceous subterranean formation. The gas flow rate through the formation is affected by many factors including the matrix porosity of the formation, the system of fractures within the formation and the stress within the carbonaceous matrix which makes up the formation.
An unstimulated solid carbonaceous subterranean formation has a natural system of fractures, the smaller and more common ones being referred to as cleats or collectively as a cleat system. To reach the wellbore, the methane must desorb from a sorption site within the matrix and diffuse through the matrix to the cleat system. The methane then passes through the cleat system to the wellbore.
The cleat system communicating with a production well often does not provide for an acceptable methane recovery rate. In general, solid carbonaceous formations require stimulation to enhance the recovery of methane from the formation. Various techniques have been developed to stimulate solid carbonaceous subterranean formations and thereby enhance the rate of methane recovery from these formations. These techniques typically attempt to enhance the desorbtion of methane from the carbonaceous matrix of the formation and enhance the permeability of the formation.
One example of a technique for stimulating the production of methane from a solid carbonaceous subterranean formation is to complete a production wellbore with an open-hole cavity. To do this, a wellbore is first drilled to a location above the solid carbonaceous subterranean formation. The wellbore may then be cased with the casing being cemented in place using a conventional drilling rig. A modified drilling rig is then used to drill an open hole interval within the formation. An "open-hole" interval is an interval within the solid carbonaceous subterranean formation which is not cased. The open-hole interval can be completed by various methods. One method utilizes an injection/blow down cycle to create a cavity within the open-hole interval. In this method air is injected into the open hole interval and then released rapidly through a surface valve causing a gas flow shear stress to overcome the formation strength in the wellbore wall. The procedure is repeated until a suitable cavity has been created. During the procedure a small amount of water can be added to selected air injections to reduce the potential for spontaneous combustion of the carbonaceous material in the formation and the like.
Techniques such as described above are considered to be known to the art and have been disclosed in U.S. Pat. No. 5,417,286 issued May 23, 1995 to Ian D. Palmer and Dan Yee and assigned to Amoco Corporation. This patent is hereby incorporated in its entirety by reference.
The use of such completions is further described in SPE 24906 "Open Hole Cavity Completions in Coalbed Methane Wells in the San Juan Basin", presented Oct. 4-7, 1992 by I. D. Palmer, M. J. Mavor, J. P. Seidle, J. L. Spitler and R. F. Volz.
The use of cavitated completions has been found to be much more effective than the use of cased wells perforated in the solid carbonaceous subterranean formation even when fracturing or other types of cased well completions are used. When the coal in the formation surrounding the wellbore in the uncased well has insufficient strength to resist movement of coal particles into the wellbore upon the production of fluids from the coal formation, the cavity can be formed by techniques such as discussed above. Unfortunately, in some instances, the formation of cavities is not readily accomplished by the production of fluids from the wellbore. Although the formations in such instances may not have great strength, they have sufficient strength to resist the movement of coal particles into the wellbore upon the production of fluids from the coal formation. In such instances it has been found difficult to initiate and complete the formation of cavities in the coal formations.
Since the use of cavities with such wellbores has been found to be much more effective than other techniques for the production of methane, a continuing effort has been directed to the development of an improved method for the stimulation of cavitated wellbores in such formations.